This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Lassa virus (LASV), an Old World Arenavirus and the etiological agent of Lassa hemorrhagic fever (LHF), causes up to 300,000 annual infections in West Africa. Approximately 30% of infections result in disease varying from mild influenza-like illness to lethal LHF causing several thousand deaths per year. The large disease burden, severe complications, lethality and the possibility that LASV can be used as a biological warfare agent make a strong case for effective vaccine development. A ML29 Lassa fever vaccine candidate carrying mutated major structural proteins of LASV and RdRp of non-pathogenic Mopeia is attenuated in guinea pigs and rhesus macaques. We have shown that ML29 vaccine protects guinea pigs from lethal infection with homologous and heterologous LASV strains. Here we report the efficacy of the vaccine in nonhuman primates. Six marmosets (Callithrix jacchus) were vaccinated with 1000 PFU of ML29 and challenged on day 30 with lethal dose of LASV-Josiah. A second group of marmosets (n=4), the unvaccinated control group, was infected with the same dose of LASV. Animals were observed for clinical signs of disease and blood was collected for hematology and blood chemistry. Prominent histopathology findings included hepatic necrosis and immunophenotypic alterations of cells in target tissues confirming immunosuppressive phenotype of fatal human LHF.